Wireless communication system using pilot subcarrier allocation

ABSTRACT

A wireless communication system using pilot subcarrier allocation and a method of allocating the pilot subcarriers for use in downlink and uplink communication in a multiple-input multiple-output (MIMO) antenna system using orthogonal frequency division multiplexing (OFDM) modulation are disclosed. The method includes providing a frame structure comprising OFDM symbols in time domain and subcarriers in frequency domain and alternately allocating first pilot subcarriers for a first antenna and second pilot subcarriers for a second antenna in the time and frequency domains, wherein each of the alternating first pilot subcarriers and the second pilot subcarriers is separated by a multiple of 9 subcarriers in the frequency domain and further allocated in two contiguous OFDM symbols.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/456,087, filed on Apr. 25, 2012, now U.S. Pat. No. 8,675,766, whichis a continuation of U.S. patent application Ser. No. 12/208,213, filedon Sep. 10, 2008, now U.S. Pat. No. 8,199,843, which claims the benefitof earlier filing date and right of priority to Korean PatentApplication Nos. 10-2008-0078433, filed on Aug. 11, 2008, and10-2007-0091730, filed on Sep. 10, 2007, and also claims the benefit ofU.S. Provisional Application Ser. No. 61/045,624, filed on Apr. 17,2008, and 61/021,894, filed on Jan. 17, 2008, the contents of which areall hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to a wireless communication system. Inparticular, the present invention relates to a method of allocatingpilot subcarriers in the wireless communication system including amultiple-input multiple-output (MIMO) antenna system.

DESCRIPTION OF THE RELATED ART

The Institute of Electrical and Electronics Engineers (IEEE) 802.16standard provides a technology of supporting a broadband wireless accessand a protocol. The standardization has been progressed since 1999 andIEEE 802.16-2001 was approved in 2001. This has been established basedon a single carrier physical layer called “WirelessMAN-SC”. In the IEEE802.16a which was approved in 2003, “WirelessMAN-OFDM” and“WirelessMAN-OFDMA” were added to a physical layer in addition to“WirelessMAN-SC”. After the IEEE 802.16a standard was completed, arevised IEEE 802.16-2004 was approved in 2004. In order to correct bugsand errors of the IEEE 802.16-2004, IEEE 802.16-2004/Cor1 was completedin the form of “corrigendum” in 2005.

A MIMO antenna technology improves data transmission/receptionefficiency using multiple transmission antennas and multiple receptionantennas. The MIMO technology was introduced in the IEEE 802.16astandard and has been continuously updated.

The MIMO technology is divided into a spatial multiplexing method and aspatial diversity method. In the spatial multiplexing method, sincedifferent data is simultaneously transmitted, data can be transmitted ata high speed without increasing the bandwidth of a system. In thespatial diversity method, since the same data is transmitted viamultiple transmission antennas so as to obtain diversity gain,reliability of the data is increased.

A receiver needs to estimate a channel in order to restore datatransmitted from a transmitter. Channel estimation indicates a processof compensating for distortion of a signal, which occurs by a rapidenvironment variation due to fading and restoring the transmissionsignal. In general, for channel estimation, the transmitter and thereceiver need to know pilots.

In the MIMO system, a signal experiences a channel corresponding to eachantenna. Accordingly, it is necessary to arrange the pilots inconsideration of multiple antennas. While the number of pilots isincreased as the number of antennas is increased, it is impossible toincrease the number of antennas so as to increase a data transfer rate.

In the prior art, different pilot allocation structures have beendesigned and used according to permutation (dispersion/AMC) methods.This is because the permutation methods are separated from each other intime domain in the IEEE 802.16e system and thus structures differentlyoptimized according to permutation methods are designed. If thepermutation methods coexist in certain time instance, a unified basicdata allocation structure is necessary.

In the prior art, since severe pilot overhead occurs, a transfer rate isdecreased. In addition, since the same pilot structure is applied toadjacent cells or sectors, collision between the cells or the sectorsmay occur. Accordingly, there is a need for a method of efficientlyallocating pilot subcarriers in the MIMO system.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of efficientlyallocating pilot subcarriers in wireless communication system includinga MIMO system. The object of the present invention can be achieved byproviding a method of allocating pilot subcarriers for a plurality ofantennas in the MIMO antenna system over a plurality of orthogonalfrequency division multiplexing (OFDM) symbols and a plurality ofsubcarriers. The method includes allocating each of the pilotsubcarriers for the plurality of antennas by the same number in everyOFDM symbols. The pilot subcarriers form pairs of two pilot subcarriersand are allocated to be alternately arranged on two contiguous OFDMsymbols in a time domain.

According to an embodiment of the present invention, a method ofallocating pilot subcarriers for use in downlink and uplinkcommunication in the MIMO antenna system using OFDM modulation isprovided. The method includes providing a frame structure comprisingOFDM symbols in time domain and subcarriers in frequency domain andallocating pilot locations according to equation:P _(i)=18k+9m _(i)+3(└n _(s)/2┘ mod 3)where, P_(i) denotes pilot index of an i^(th) antenna, k=0, 1, . . . ,N_(pilot), i=(n_(s)+i) mod 2, and └n┘ denotes an integer smaller than n.

According to an embodiment of the present invention, the method ofallocating pilot subcarriers for use in at least one of downlink anduplink communication in the MIMO antenna system using OFDM modulationincludes providing a frame structure comprising OFDM symbols in timedomain and subcarriers in frequency domain and

alternately allocating first pilot subcarriers for a first antenna andsecond pilot subcarriers for a second antenna in at least one of thetime and frequency domains, wherein each of the alternating first pilotsubcarriers and the second pilot subcarriers are separated by a multipleof a first predetermined number of subcarriers in the frequency domainand further allocated in two contiguous OFDM symbols, wherein the firstpredetermined number is 9.

In an aspect of the present invention, each of the first pilotsubcarriers separated by two OFDM symbols is shifted by a secondpredetermined number of subcarriers, and each of the second pilotsubcarriers separated by two OFDM symbols is shifted by the secondpredetermined number of subcarriers to differentiate frequencyselectivity, wherein the second predetermined number is a multiple of 3.Each OFDM symbol comprises first and second pilot subcarriers.Preferably, number of each of the first and second pilot subcarriers ineach OFDM symbol is the same. Preferably, the second predeterminednumber is 3. The frame structure is used in one of uplink and downlinkcommunications.

In another aspect of the present invention, the method further includesalternately allocating third pilot subcarriers for a third antenna andfourth pilot subcarriers for a fourth antenna in the time and frequencydomains, wherein each of the alternating third pilot subcarriers and thefourth pilot subcarriers are separated by the multiple of the firstpredetermined subcarriers in the frequency domain and further allocatedin two contiguous OFDM symbols. Preferably, the first pilot subcarriersare adjacent to the third pilot subcarriers in the frequency domain andthe second pilot subcarriers are adjacent to the fourth pilotsubcarriers in the frequency domain. Preferably, the first pilotsubcarriers and the third pilot subcarriers are separated by at leastone subcarrier in the frequency domain and the second pilot subcarriersand the fourth pilot subcarriers are separated by at least onesubcarrier in the frequency domain.

In one aspect of the present invention, the method may further includealternately allocating fifth pilot subcarriers for a fifth antenna andsixth pilot subcarriers for a sixth antenna in the time and frequencydomains, wherein each of the alternating fifth pilot subcarriers and thesixth pilot subcarriers are separated by the multiple of the firstpredetermined subcarriers in the frequency domain and further allocatedin two contiguous OFDM symbols and alternatively allocating seventhpilot subcarriers for a seventh antenna and eighth pilot subcarriers fora eighth antenna in the time and frequency domains, wherein each of thealternating seventh pilot subcarriers and the eighth pilot subcarriersare separated by the multiple of the first predetermined subcarriers inthe frequency domain and further allocated in two contiguous OFDMsymbols, wherein the fifth pilot subcarriers are adjacent to andpositioned between the third and seventh pilot subcarriers in thefrequency domain and the sixth pilot subcarriers are adjacent to andpositioned between the fourth and eighth pilot subcarriers in thefrequency domain. In another aspect of the present invention, the fifthpilot subcarriers and the seventh pilot subcarriers are separated by atleast one subcarrier in the frequency domain and the sixth pilotsubcarriers and the eighth pilot subcarriers are separated by at leastone subcarrier in the frequency domain.

The frame structure may be used in a type of a full usage of subchannels(FUSC) permutation mode. The frame structure may be used in a type of anadaptive modulation and coding (AMC) permutation mode. Preferably, astarting location of the first pilot subcarriers in a first OFDM symbolis offset by 1 subcarrier.

According to an embodiment of the present invention, a wirelesscommunication system using OFDM modulation for downlink and uplinkcommunication includes a MIMO antenna, an OFDM modulator operablyconnected to the MIMO antenna, and a processor operably connected to theOFMD modulator. The processor is configured to provide a frame structurecomprising OFDM symbols in time domain and subcarriers in frequencydomain and allocating pilot locations according to equation:P _(i)=18k+9m _(i)+3(└n _(s)/2┘ mod 3)where, P_(i) denotes pilot index of an i^(th) antenna, k=0, 1, . . . ,N_(pilot), i=(n_(s)+i) mod 2, and └n┘ denotes an integer smaller than n.

According to an embodiment of the present invention, in the wirelesscommunication system using OFDM modulation for downlink and uplinkcommunication and including a MIMO antenna, an OFDM modulator operablyconnected to the MIMO antenna, and a processor operably connected to theOFDM modulator, the processor is configured to provide a frame structurecomprising OFDM symbols in time domain and subcarriers in frequencydomain and alternately allocate first pilot subcarriers for a firstantenna and second pilot subcarriers for a second antenna in at leastone of the time and frequency domains, wherein each of the alternatingfirst pilot subcarriers and the second pilot subcarriers is separated bymultiple of a first predetermined number of subcarriers in the frequencydomain and further allocated in two contiguous OFDM symbols, wherein thefirst predetermined number is 9. Preferably, each of the first pilotsubcarriers separated by two OFDM symbols is shifted by a secondpredetermined number of subcarriers, and each of the second pilotsubcarriers separated by two OFDM symbols is shifted by the secondpredetermined number of subcarriers to differentiate frequencyselectivity, wherein the second predetermined number is a multiple of 3.

In an aspect of the present invention, the processor includes asubcarrier allocator allocating symbols and pilots to the subcarriersfor use in at least one of the downlink and uplink communication. Theprocessor may further include a channel encoder encoding an input streamand building a coded word, a mapper mapping the coded word to a symbolrepresenting a position on signal constellation, and a MIMO processorprocessing the symbol.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

FIG. 1 is a block diagram of a transmitter having multiple antennas.

FIG. 2 is a block diagram of a receiver having multiple antennas.

FIG. 3 shows a frame structure.

FIG. 4 shows a pilot arrangement of two transmission antennas in partialusage of subchannels (PUSC).

FIG. 5 shows a pilot arrangement of two transmission antennas in fullusage of subchannels (FUSC).

FIG. 6 shows a pilot arrangement of four transmission antennas in thePUSC.

FIG. 7 shows a pilot arrangement of four transmission antennas in FUSC.

FIG. 8 is a graph showing cases of Table 2 by a ratio of pilot overheadand a ratio of guard subcarriers.

FIG. 9 shows the pilot arrangement of the two transmission antennas.

FIG. 10 shows the pilot arrangement of the four transmission antennas.

FIG. 11 illustrates the pilot arrangement of three or four transmissionantennas by equation.

FIG. 12 shows the pilot arrangement of the two transmission antennas.

FIG. 13 shows the pilot arrangement of the two transmission antennas.

FIG. 14 shows the pilot arrangement of the four transmission antennas.

FIG. 15 shows the pilot arrangement of the four transmission antennas.

FIGS. 16˜18 show some pilot subcarrier allocation structures in a systemusing eight antennas according to an embodiment of the presentinvention.

FIGS. 19 and 20 show embodiments of the present invention in which apilot subcarrier allocation offset varies according to cells in a 4-Txsystem and an 8-Tx system.

FIG. 21 shows another embodiment of the 8-Tx system shown in FIG. 20.

FIG. 22 shows a pilot subcarrier allocation pattern according to anembodiment of the present invention.

FIGS. 23˜25 show pilot subcarrier allocation patterns in a 2-Tx systemaccording to an embodiment of the present invention.

FIGS. 26 and 27 show pilot subcarrier allocation patterns in a 4-Txsystem according to an embodiment of the present invention.

FIG. 28 shows a pilot subcarrier allocation pattern in a 4-Tx systemaccording to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description, reference is made to theaccompanying drawing figures which form a part hereof, and which show byway of illustration specific embodiments of the invention. It is to beunderstood by those of ordinary skill in this technological field thatother embodiments may be utilized, and structural, electrical, as wellas procedural changes may be made without departing from the scope ofthe present invention. Wherever possible, the same reference numberswill be used throughout the drawings to refer to the same or similarparts.

The following technology may be used in various wireless communicationsystems. A wireless communication system is widely provided in order toprovide various communication services such as voice and packet data.This technology may be used in downlink or uplink. In general, thedownlink indicates communication from a base station (BS) to a userequipment (UE) and the uplink indicates communication from the UE to theBS. The BS generally indicates a fixed station communicating with the UEand may also be called a node-B, a base transceiver system (BTS) or anaccess point. The UE may be fixed or moved and may also be called amobile station (MS), a user terminal (UT), a subscriber station (SS) ora wireless device.

Hereinafter, an efficient pilot structure for a new system will bedescribed. The new system will be described concentrating on an IEEE802.16m system, but the same principle may apply to other systems.

A communication system may be a multiple-input multiple-output (MIMO)system or a multiple-input single-output (MISO) system. The MIMO systemuses a plurality of transmission antennas and a plurality of receptionantennas. The MISO system uses a plurality of transmission antennas andone reception antenna.

FIG. 1 is a block diagram of a transmitter having multiple antennas.Referring to FIG. 1, the transmitter 100 includes a channel encoder 120,a mapper 130, an MIMO processor 140, a subcarrier allocator 150 and anorthogonal frequency division multiplexing (OFDM) modulator 160. Thechannel encoder 120, mapper 130, MIMO processor 140, and subcarrierallocator 150 may be embodied as separate components or combined in asingle processor of the transmitter 100.

The channel encoder 120 encodes an input stream according to apredefined coding method and builds a coded word. The mapper 130 mapsthe coded word to a symbol representing a position on signalconstellation. A modulation scheme of the mapper 130 is not restrictedand may include an m-phase shift keying (m-PSK) scheme or anm-quadrature amplitude modulation (m-QAM) scheme.

The MIMO processor 140 processes the input symbol by a MIMO method usinga plurality of transmission antennas 190-1, . . . , and 190-Nt. Forexample, the MIMO processor 140 can perform precoding based on codebook.

The subcarrier allocator 150 allocates the input symbol and pilots tosubcarriers. The pilots are arranged according to the transmissionantennas 190-1, . . . , and 190-Nt. The pilots are known by both thetransmitter 100 and a receiver (200 of FIG. 2), which are used for thechannel estimation or the data demodulation, and are also calledreference signals.

The OFDM modulator 160 modulates the input symbol and outputs OFDMsymbols. The OFDM modulator 160 may perform an inverse fast Fouriertransform (IFFT) with respect to the input symbol and further insert acyclic prefix (CP) after performing the IFFT. The OFDM symbols aretransmitted via the transmission antennas 190-1, . . . , and 190-Nt.

FIG. 2 is a block diagram of a receiver having multiple antennas.Referring to FIG. 2, the receiver 200 includes an OFDM demodulator 210,a channel estimator 220, an MIMO post-processor 230, a demapper 240 anda channel decoder 250. The channel estimator 220, MIMO post-processor230, demapper 240 and channel decoder 250 may be embodied as separatecomponents or combined in a single processor of the receiver 200.

Signals received via reception antennas 290-1, . . . , and 290-Nr areFast Fourier transformed (FFT) by the OFDM demodulator 210. The channelestimator 220 estimates channels using pilots. The MIMO post-processor230 performs a post-process corresponding to the MIMO processor 140. Thedemapper 240 demaps the input symbol to the coded word. The channeldecoder 250 decodes the coded word and restores original data.

FIG. 3 is an example of a frame structure. A frame is a data sequenceduring a fixed time period used by a physical specification, whichrefers to section 8.4.4.2 of IEEE standard 802.16-2004 “Part 16: AirInterface for Fixed Broadband Wireless Access Systems” (hereinafter,referred to as reference Document 1).

Referring to FIG. 3, the frame includes a downlink (DL) frame and anuplink (UL) frame. Time division duplex (TDD) is a scheme in which theuplink and downlink transmission are separated in time domain but sharethe same frequency. Normally, the DL frame precedes the UL frame. The DLframe starts in the order of Preamble, Frame Control Header (FCH),Downlink (DL)-MAP, Uplink (UL)-MAP and burst regions (DL burst #1˜5 andUL burst #1˜5). A guard time for separating the DL frame and the ULframe from each other is inserted at both an intermediate portion of theframe (between the DL frame and the UL frame) and a last portion of theframe (following the UL frame). A transmit/receive transition gap (TTG)is a gap defined between a downlink burst and a subsequent uplink burst.A receive/transmit transition gap (RTG) is a gap defined between anuplink burst and a subsequent downlink burst.

The preamble is used for initial synchronization between the BS and theUE, cell search, frequency offset estimation and channel estimation. TheFCH includes information regarding the length of the DL-MAP message andthe coding scheme of the DL-MAP. The DL-MAP is a region where the DL-MAPmessage is transmitted. The DL-MAP message defines the access of adownlink channel. The DL-MAP message includes a configuration changecount of a Downlink Channel Descriptor (DCD) and a BS identifier (ID).The DCD describes a downlink burst profile applied to a current frame.The downlink burst profile refers to the property of a downlink physicalchannel, and the DCD is periodically transmitted by the BS through theDCD message.

The UL-MAP is a region where the UL-MAP message is transmitted. TheUL-MAP message defines the access of an uplink channel. The UL-MAPmessage includes a configuration change count of an Uplink ChannelDescriptor (UCD), and an effective start time of the uplink allocationdefined by the UL-MAP. The UCD describes an uplink burst profile. Theuplink burst profile refers to the property of an uplink physicalchannel, and the UCD is periodically transmitted by the BS through theUCD message.

Hereinafter, a slot is a minimum data allocation unit and is defined bya time and a subchannel. The number of subchannels depends on a FFT sizeand time-frequency mapping. The subchannel includes a plurality ofsubcarriers and the number of subcarriers per subchannel variesaccording to a permutation method. The permutation indicates mapping ofa logical subchannel to a physical subcarrier. The subchannel includes48 subcarriers in full usage of subchannels (FUSC) and the subchannelincludes 24 or 16 subcarriers in partial usage of subchannels (PUSC). Asegment indicates at least one subchannel set.

In order to map data to physical subcarriers in a physical layer, twosteps are generally performed. In a first step, data is mapped to atleast one data slot on at least one logical subchannel. In a secondstep, the logical subchannel is mapped to the physical subchannel. Thisis called permutation. Reference document 1 discloses a permutationmethod such as FUSC, PUSC, Optimal-FUSC (O-FUSC), Optional-PUSC (O-PUSC)and Adaptive modulation and coding (AMC). A set of OFDM symbols usingthe same permutation method is called a permutation zone and one frameincludes at least one permutation zone.

The FUSC and the O-FUSC are used only for the downlink transmission. TheFUSC is composed of one segment including all subchannel groups. Thesubchannels are mapped to the physical subcarriers distributed via allphysical channels. The mapping is changed according to OFDM symbols. Theslot is composed of one subchannel on one OFDM symbol. The methods ofallocating pilots in the O-FUSC and FUSC are different from each other.

The PUSC is used for both the downlink transmission and the uplinktransmission. In the downlink, each physical channel is divided intoclusters including 14 contiguous subcarriers on two OFDM symbols. Thephysical channel is mapped in the unit of six groups. In each group, thepilots are allocated to the clusters at fixed positions. In the uplink,the subcarriers are divided into tiles composed of four contiguousphysical subcarriers on three OFDM symbols. The subchannel includes sixtiles. The pilots are allocated to corners of the tiles. The O-PUSC isused only for the uplink transmission and the tile is composed of threecontiguous physical subcarriers on three OFDM symbols. The pilots areallocated to the centers of the tiles.

FIGS. 4 and 5 show a pilot arrangement of two transmission antennas inthe PUSC and FUSC, respectively. FIGS. 6 and 7 show a pilot arrangementof four transmission antennas in the PUSC and FUSC, respectively. Theyrefer to section 8.4.8.1.2.1.1, section 8.4.8.1.2.1.2, section8.4.8.2.1, and section 8.4.8.2.2 of IEEE standard 802.16-2004/Cor1-2005“Part 16: Air Interface for Fixed and Mobile Broadband Wireless AccessSystems; Amendment 2: Physical and Medium Access Control Layers forCombined Fixed and Mobile Operation in Licensed Bands and Corrigendum 1”(hereinafter, referred to as Reference Document 2).

Referring to FIGS. 4 to 7, pilot overhead is large when the allocationof the subcarrier is performed according to the PUSC or the FUSC. Inparticular, the overhead is larger when one transmission antenna is usedcompared to when at least two transmission antennas is used, inconsideration of pilot overhead per transmission antenna.

Table 1 shows the pilot overhead according to the number of transmissionantennas in each permutation method.

TABLE 1 Number of transmission antennas PUSC FUSC O-FUSC 1  14.28%(14.28%) 9.75% (9.75%) 11.1% (11.1%) 2 14.28% (7.14%) 9.75% (4.78%)11.1% (5.55%) 4 28.55% (7.14%) 18.09% (4.52%)  22.21% (5.55%) 

The pilot overhead is a value obtained by dividing the number ofsubcarriers allocated to the pilots by the number of all subcarrierswhich are used. The value in parenthesis indicates the pilot overheadper transmission antenna. Further, according to Reference Document 2, iffour or three transmission antennas are used, mapping of the data to thesubchannels is performed after puncturing or truncation with respect tochannel encoded data.

Now, a pilot structure for multiple antennas according to the presentinvention will be described. Design criteria for an optimal pilotstructure are as follows.

(1) The pilot overhead in a single antenna is about 4 to 9% in atime-frequency domain. (2) One slot may include 48 subcarriers on twocontiguous OFDM symbols. (3) The pilot subcarriers are uniformlydistributed in the time-frequency domain. (4) The pilot overhead pertransmission antenna is similarly maintained and the total pilotoverhead is increased as the number of transmission antennas isincreased. In consideration of the total pilot overhead, the total pilotoverhead is equally maintained so as not to exceed 20% in at least threetransmission antennas.

(5) Even when the number of transmission antennas is increased, mappingof the data to the subchannels is not influenced.

In order to derive a possible candidate group using the above-describedcondition, the following requirements can be expressed by equations andthe candidate group satisfying the requirements can be derived.(Nused−2*Np)mod Nsub=0  (1)(Nused−2*Np)mod Nsub=0  (2)(Nused−Np)mod Nsub=0  (3)0.04<=Np<=0.09  (4)Ng=Nfft−Nused−1(DC subcarrier)  (5)(Nused−Np)/Nsub.sym>=Nsch.pusc  (6)where, Nused denotes the number of used subcarriers, Np denotes thenumber of pilot subcarriers, Ng denotes the number of guard subcarriers,NM denotes a FFT size, Nsub denotes the number of subcarriers persubchannel, Nsub.sym denotes the number of subcarriers allocated to thesubchannel on one OFDM symbol, and Nsch.pusc denotes the number ofsubchannels which can be generated in the existing DL-PUSC.

Table 2 shows 26 candidate groups according to the design criteria.

TABLE 2 # of # of # of Pilot FFT Used Pilot Guard Subch - 1 Subch - 2Subch - 4 Overhead -1 Data Guard Pilot # Size SCs SCs SCs antennaantennas antennas antenna Ratio Ratio Spacing  (1) 2048 1656 72 391 6663 60 4.3478 80.859 19.092 11.5  (2) 2048 1656 96 391 65 61 57 5.797180.859 19.092 8.625  (3) 2048 1680 72 367 67 64 61 4.2857 82.031 17.9211.6667  (4) 2048 1680 96 367 66 62 58 5.7143 82.031 17.92 8.75  (5)2048 1680 120 367 65 60 55 7.1429 82.031 17.92 7  (6) 2048 1704 72 34368 65 62 4.2254 83.203 16.748 11.833  (7) 2048 1704 96 343 67 63 595.6338 83.203 16.748 8.875  (8) 2048 1704 120 343 66 61 56 7.0423 83.20316.748 7.1  (9) 2048 1728 72 319 69 66 63 4.1667 84.375 15.576 12 (10)2048 1728 96 319 68 64 60 5.5556 84.375 15.576 9 (11) 2048 1728 120 31967 62 57 6.9444 84.375 15.576 7.2 (12) 2048 1728 144 319 66 60 54 8.333384.375 15.576 6 (13) 2048 1752 72 295 70 67 64 4.1096 85.547 14.40412.1667 (14) 2048 1752 96 295 69 65 61 5.4795 85.547 14.404 9.125 (15)2048 1752 120 295 68 63 58 6.8493 85.547 14.404 7.3 (16) 2048 1752 144295 67 61 55 8.2192 85.547 14.404 6.08 (17) 2048 1776 72 271 71 68 654.0541 86.719 13.232 12.333 (18) 2048 1776 96 271 70 66 62 5.4054 86.71913.232 9.25 (19) 2048 1776 120 271 69 64 59 6.7568 86.719 13.232 7.4(20) 1024 816 48 207 32 30 28 5.8824 79.688 20.215 8.5 (21) 1024 840 48183 33 31 29 5.7143 82.031 17.871 8.75 (22) 1024 864 48 159 34 32 305.5556 84.375 15.527 9 (23) 1024 864 72 159 33 30 27 8.3333 84.37515.527 6 (24) 1024 888 48 135 35 33 31 5.4054 86.719 13.184 9.25 (25)1024 888 72 135 34 31 28 8.1081 86.719 13.184 6.1665 (26) 512 432 24 7917 16 15 5.5556 84.375 15.43 9

The used subcarriers (used SCs), the pilot subcarriers (Pilot SCs) andthe guard subcarriers (Guard SCs) are obtained according to the FFTsize, and the subchannel according to the number of transmissionantennas is obtained. The used subcarrier is a value excluding a DCsubcarrier. For example, in the case (1), the number of subchannels inone transmission antenna is (the number of used subcarriers—the numberof pilot subcarriers)/(the number of subcarriers allocated to thesubchannels on one OFDM symbol)=1656−72/24=66.

FIG. 8 is a graph showing cases of Table 2 by a ratio of pilot overheadand a ratio of guard subcarriers. Referring to FIG. 8, among cases (1)to (26) shown in Table 2, in consideration of the system bandwidth orthe FFT size, it can be seen that cases which are most suitable for 5 to20 MHz or 512 to 2048 FFT size in the same design criteria are cases(10), (22) and (26). In Table 2, the cases (10), (22) and (26) have aspacing of 9 pilots. That is, in one embodiment of the presentinvention, the arrangement of the pilots at a spacing of 9 subcarriersis provided.

Table 3 shows the subcarrier allocation.

TABLE 3 Value Parameter 2048 1024 512 Comments # of DC 1 1 1 Index1024/512/256 Subcarriers (counting from 0) # of Guard 160 80 40 —Subcarriers, Left # of Guard 159 79 39 — Subcarriers, Right # of Used1729 865 433 Number of all subcar- Subcarriers riers used within a(N_(used)) symbol, including pilots & a DC carrier # of Pilot 96 48 24 —Subcarriers (N_(pilot)) Pilot Subcarrier 18k + 9m, + n_(s) denotes asymbol Index 3(└n_(s)/2┘mod 3) index, in which index ‘0’ k = 0, 1, . . ., N_(pilot) should be the first m_(i) = (n_(s) + i) mod 2, symbol of thecurrent i = 0, 1 zone. DC carrier is excluded for the calculation of thepilot subcarrier index. # of Data 1632 816 408 — Subcarriers (24) (24)(24) (in each symbol per subch.)

The subchannels are mapped to subcarriers excluding the subcarriers usedafter allocating the pilot subcarriers. At this time, a general PUSC orFUSC permutation method is applicable.

According to the above mentioned method, it is possible to improvethroughput by 6 to 13% in the PUSC/FUSC. For example, while 60subchannels can be obtained in the PUSC and 64 subchannels can beobtained in the FUSC according to the prior art, 68 subchannels can beobtained according to the above mentioned method. If a new permutationmethod is applied in consideration of the multiple antennas, capabilitycan be prevented from deteriorating due to data puncturing ortruncation.

The equation shown in the pilot subcarrier index of Table 3 represents apilot index Pi of an i^(th) antenna. This is expressed by the followingequation again.P _(i)=18k+9m _(i)+3(└n _(s)/2┘ mod 3)where, k=0, 1, . . . , Npilot, mi=(ns+i) mod 2, i=0, 1, └n┘ denotes aninteger smaller than n. In Equation 1, a factor “18” indicates that thenumber of subcarriers of the subchannel on one OFDM symbol is 18, afactor “9” indicates that the pilot subcarriers are arranged at aspacing of 9 subcarriers, and a factor “3” indicates that thesubcarriers are shifted at a spacing of three subcarriers in the unit ofa slot. In this case, each slot occupies 2 consecutive OFDM symbols intime domain.

FIG. 9 is an example of the pilot arrangement of two transmissionantennas. Referring to FIG. 9, one slot includes 72 subcarriers on twocontiguous OFDM symbols, and the pilot subcarriers for a first antenna(antenna 0) and the pilot subcarriers for a second antenna (antenna 1)are arranged at a spacing of 9 subcarriers on each OFDM symbol. Inaddition, on the first OFDM symbol and the second OFDM symbol, the pilotsubcarriers for the first antenna (antenna 0) and the pilot subcarriersfor the second antenna (antenna 1) are alternately arranged.

In a second slot, the pilot subcarriers are shifted from the pilotsubcarriers allocated to the first slot by three subcarriers. In a thirdslot, the pilot subcarriers are shifted from the pilot subcarriersallocated to the second slot by three subcarriers. As a result, the samepilot arrangement is repeated in every three slots.

In the pilot arrangement, the pilots can be moved at a constant spacingin the time domain or the frequency domain, and thus, do not haveabsolute positions. The pilot subcarriers may be shifted by at aconstant time interval or/and a subcarrier spacing while a spacingbetween the pilot subcarriers is maintained.

FIG. 10 is an example of the pilot arrangement of four transmissionantennas. Referring to FIG. 10, the pilot subcarriers for fourtransmission antennas (antenna 0, antenna 1, antenna 2, and antenna 3)are contiguous in the frequency domain or in the time domain. The pilotsubcarriers for the transmission antennas are arranged at a spacing of12 subcarriers.

In a second slot, the pilot subcarriers are shifted from the pilotsubcarriers allocated to the first slot by six subcarriers. As a result,the same pilot arrangement is repeated every two slots.

In the four transmission antennas, since the number of pilots is largerthan the number of the two transmission antennas, the pilot spacing iswider than the pilot spacing in the two transmission antennas and thecyclic period of the repeated slot is decreased.

FIG. 11 illustrates the pilot arrangement of three or four transmissionantennas by equation. Referring to FIG. 11, G₀, G₁, G₂, and G₃ aredefined in a 2×2 region including two OFDMs (i=0, 1) and twosubcarriers. For example, in the four transmission antennas, the pilotcarriers are arranged as shown in Table 4, which is equal to thearrangement of FIG. 10.

TABLE 4 Antenna 0 Antenna 1 Antenna 2 Antenna 3 Pilot Subcarrier G₀ G₁G₂ G₃ Index

In the three transmission antennas, the pilot subcarriers are arrangedas shown in Table 5.

TABLE 5 └n₂/2┘ mod2 0 1 2 Pilot Antenna 0 G₀, G₃ G₂ G₁ SubcarrierAntenna 1 G₁ G₀, G₃ G₂ Index Antenna 2 G₂ G₁ G₀, G₃

In order to express the pilot arrangement for the four transmissionantennas by equation, six pilot subcarriers P^(i) ₀, P^(i) ₂, P^(i) ₃,P^(i) ₄, and P^(i) ₅ are considered on OFDM symbols (i=0,1), as shown inTable 6.

TABLE 6 m_(i) = 0 m_(i) = 1 k = even P^(i) ₀ = {PilotSub(k, m_(i))}P^(i) ₂ = {PilotSub(k, m_(i)) + 3} P^(i) ₁ = {PilotSub(k, m_(i)) + 1} k= odd P^(i) ₃ = {PilotSub(k, m_(i)) − 5} P^(i) ₄ = {PilotSub(k, m_(i)) −3} P^(i) ₅ = {PilotSub(k, m_(i)) − 2}where, PilotSub(k,m_(i))=18k+9m_(i)+6(└n_(s)/2┘ mod 2).

Accordingly, in one slot, a pilot subcarrier set may be mapped to thefour pilot subcarriers G₀, G₁, G₂, and G₃ as shown in Equation 2.G _(i) =P ^(i) ₀ +P ^(i) ₂ +P ^(i) ₄ and G _(i+2) =P ^(i) ₁ +P ^(i) ₃ +P^(i) ₅  Equation 2

where, i=0,1.

According to the pilot structure, the pilot overhead satisfying thedesign criteria is obtained and the overhead is decreased from the priorart by 5% or more. The pilot overhead is 5.55% in the two transmissionantennas, 5.55% in the three transmission antennas, and 4.16% in thefour transmission antennas.

Even when the number of transmission antennas is increased, the mappingof data to the subchannels is not influenced. Accordingly, thepermutation method can be simply performed.

In a method of generating distributed subchannels used in the existingIEEE 802.16-2004 standard, pilot subcarriers for the first antenna andthe second antenna are first allocated and subchannels are constructedusing residual subcarriers. In a third antenna and a fourth antenna,since the pilot subcarriers are allocated and used using the allocatedsubchannels, the subchannels of the same number are constructedregardless of the number of antennas. However, according to the pilotstructure, a necessary number of pilot subcarriers are allocatedaccording to the number of antennas and the subchannels are constructedusing the residual subcarriers. Accordingly, the number of subchannelsis increased while optimizing the pilot overhead.

FIG. 12 is another example of the pilot arrangement of two transmissionantennas. Referring to FIG. 12, the pilot subcarriers for one antennaare arranged on one ODFM symbol at a spacing of nine subcarriers. Thatis, the pilot subcarriers for the first antenna (antenna 0) are arrangedon the first OFDM symbol at a spacing of nine subcarriers, and the pilotsubcarriers for the second antenna (antenna 1) are arranged on thesecond OFDM symbol at a spacing of nine subcarriers.

In a second slot, the pilot subcarriers are arranged by shifting thepilot subcarriers allocated to the first slot by three subcarriers. In athird slot, the pilot subcarriers are arranged by shifting the pilotsubcarriers allocated to the second slot by three subcarriers. As aresult, the same pilot arrangement is repeated every three slots.

FIG. 13 is another example of the pilot arrangement of two transmissionantennas. Referring to FIG. 13, the arrangement of the pilot subcarriersin one slot is equal to that of the embodiment shown in FIG. 9. However,in a second slot, the pilot subcarriers are shifted from the pilotsubcarriers by four subcarriers. As a result, the same pilot arrangementis repeated every two slots.

Referring to FIG. 14, the pilot subcarriers for the transmissionantennas (antenna 0, antenna 1, antenna 2, and antenna 3) are arrangedat a spacing of 12 subcarriers and are arranged by shifting thefrequency position in the unit of two OFDM symbols in every slot. In asecond slot, the pilot subcarriers are shifted from the pilotsubcarriers allocated to the first slot by six subcarriers. As a result,the same pilot arrangement is repeated every two slots.

FIG. 15 is another example of the pilot arrangement of four transmissionantennas (antenna 0, antenna 1, antenna 2, and antenna 3). Referring toFIG. 15, the pilot subcarriers for the transmission antennas arearranged at a spacing of 12 subcarriers and the pilot subcarriers forthe antennas are alternately arranged between two subsequent OFDMsymbols in one slot. By alternately arranging 2 pilot subcarriers in twosubsequent OFDM symbols, that is, by equally allocating each of pilotsubcarriers for each antenna in 1 OFDM symbol, transmission power foreach antenna can be balanced in certain time instance. In FIG. 15, twopilot subcarriers for respective two antennas are paired, and each ofthe two pairs of pilot subcarriers are alternatively allocated in twoOFDM symbols.

Hereinafter, an efficient pilot allocation structure according to anembodiment of the present invention is described. A structure forallocating the pilot subcarriers efficiently shifts the pilot structurebetween contiguous cells in order to avoid collision between pilotsubcarriers.

In the following embodiment of the present invention, as an example, abasic resource block unit is composed of 18 subcarriers (verticalaxis)*six OFDM symbols (horizontal axis). However, the method ofallocating the pilot subcarriers according to the present invention isalso applicable to other basic resource block units by expanding thesame method to a subframe or a frame.

In the following embodiment, the horizontal axis denotes a set of OFDMsymbols in the time domain and the vertical axis denotes the subcarriersin the frequency domain. P1, P2, P3, P4, P5, P6, P7 and P8 denote thepilot subcarriers corresponding to the antennas 1, 2, 3, 4, 5, 6, 7 and8, respectively.

FIGS. 16˜18 are pilot subcarrier allocation structures in a system usingeight antennas according to an embodiment of the present invention. Thepilots of the first transmission antenna (P1) and the pilots of thesecond transmission antenna (P2), the pilots of the third transmissionantenna (P3) and the pilots of the fourth transmission antennas (P4),the pilots of the fifth transmission antenna (P5) and the pilots of thesixth transmission antennas (P6), and the pilots of the seventhtransmission antenna (P7) and the pilots of the eighth transmissionantennas (P8) are respectively paired with each other and allocated soas to be contiguous to each other on two OFDM symbols, as shown in FIGS.16˜18. Further, the structure for consecutively allocating the pilotsubcarriers for the antennas at a spacing of 18 subcarriers in thefrequency axis is provided. That is, the pilot subcarriers are allocatedat a spacing of a subchannel including 18 subcarriers.

Specifically, FIG. 16 shows a pilot pattern shifted and allocated at thesubcarrier spacing of 2 per unit of two OFDM symbols. FIG. 17 shows apilot pattern shifted and allocated at the subcarrier spacing of 6 perunit of two OFDM symbols. FIG. 18 shows a pilot pattern shifted andallocated at the subcarrier spacing of 6 per unit of two OFDM symbols,as in FIG. 17, but having additional 1 subcarrier offset.

In an aspect of the present invention, the pilot subcarriers for theantennas are equally allocated in each OFDM symbol, and the pilotsubcarrier pattern is shifted and allocated at a predetermined spacingin a unit of two OFDM symbols. In another aspect of the presentinvention, the pilots for eight transmission antennas may be allocatedto be shifted by a predetermined subcarrier spacing per each of thepilots for the antennas or pilot subcarrier pairs, instead of beingcontiguously allocated, as shown in FIGS. 16˜18.

The pilot allocation may have the same allocation structure regardlessof a regular subframe or an irregular subframe. A part of the pilotsallocated according to an embodiment of the present invention may beused in a common pilot and the other part thereof may be used in adedicated pilot. All the allocated pilots may be applied to thededicated pilot or the common pilot. This embodiment is characterized inthat the pilots for all antennas are equally allocated in one OFDMsymbol for power balancing of the antenna per OFDM symbol. In the methodof allocating the pilot subcarriers according to the above-describedembodiment, a shift offset for pilot allocation may vary according tocells.

FIGS. 19 and 20 show that a pilot subcarrier allocation offset variesaccording to cells in a 4-Tx system and an 8-Tx system. In particular,FIG. 19 shows that the pilot subcarrier allocation offset is differentlyset to cells A, B and C in the 4-Tx system. And FIG. 20 shows that thepilot subcarrier allocation offset is differently set to the cells A, Band C in the 8-Tx system.

That is, the pilot allocation structure can be differently applied tothe cells A, B and C. The applied shift offset value is applied in thenumber of subcarriers of 1 to 18. The 18 subcarriers correspond to thebasic resource block size.

The shift offset value may be an integral multiple of the basic resourceblock size. Although the shift offset value is applied to the frequencyaxis in the present embodiment, it may be applied to the time axis.

If the number of contiguous cells is three or more and theabove-described principle is expanded and applied, the structures shownin FIGS. 19 and 20 may be repeatedly used or may be applied in a stateof being shifted by a predetermined subcarrier offset or a predeterminedOFDM symbol offset.

FIG. 21 shows another embodiment of the 8-Tx system shown in FIG. 20.FIG. 22 shows a pilot subcarrier allocation pattern according to anotherembodiment of the present invention.

In the eight transmission antennas, the pilots may not be allocated forall eight antennas due to the pilot overhead. That is, when the pilotsare allocated only for the antennas 1, 2, 3 and 4, the overhead can bereduced. For example, such pilot allocation structure is applicable to aSFBC-CDD method. In the pilot allocation pattern shown in FIG. 22, amethod of shifting and using the pilot pattern between contiguous cellsis equal to that of the above-described embodiment.

FIGS. 23˜25 show pilot subcarrier allocation patterns in a 2-Tx systemaccording to another embodiment of the present invention. The principleaccording to the above-described embodiment is equally applied to thepilot patterns shown in FIGS. 23˜25. Thus, the pilot subcarriers for thefirst antenna and the pilot subcarriers for the second antenna formpairs and are contiguously arranged in two contiguous OFDM symbol areas.The pilot subcarriers for all antennas are set to be included in each ofthe OFDM symbol areas by the same number in order to equally set thetransmission power allocated to the antennas at a specific time.

Preferably, the pilot subcarrier pairs are allocated at a spacing ofnine subcarriers, in consideration of optimal granularity for the pilotsubcarrier allocation in a coherence bandwidth. Further, the pairs ofthe pilot subcarriers for the first antenna and the pilot subcarriersfor the second antenna are shifted by a predetermined number ofsubcarriers in a unit of two OFDM symbols. Although FIGS. 23 and 24 showthat the shifted subcarrier spacing is a spacing corresponding to threesubcarriers, for example, the subcarrier indexes are increased by threesubcarriers, the shifted subcarrier spacing may be a predeterminedmultiple of three subcarriers, for example, six subcarriers, and may beapplied such that the indexes are decreased by three subcarrier indexes.FIG. 25 shows that the shifted subcarrier spacing is set to be 6subcarriers. FIG. 25 may also be construed that the shifted subcarrierspacing is applied such that the indexes are decreased by 3 subcarrierindexes.

As described above, the reason why the frequency-domain shift is set ata spacing corresponding to three subcarriers or the multiple of threesubcarriers is because the pilot subcarriers are allocated in a unit ofnine subcarriers and the pilot subcarrier allocation pattern is repeatedwith a predetermined period. The pilot pattern shown in FIGS. 23˜25 maybe equally and repeatedly applied to the time/frequency domain in theframe or the subframe. The positions of the pilots for the antenna 1 andthe positions of the pilots for the antenna 2 may be exchanged withoutdeparting from the principal of this embodiment.

In the pilot allocation structure shown in FIG. 23, the pilot allocationindexes for the antennas are represented in detail as follows.

<Pilot Allocation Index for FIG. 23>

Antenna 1—

18k+1 when s is 0

18k+10 when s is 1

18k+4 when s is 2

18k+13 when s is 3

18k+7 when s is 4

18k+16 when s is 5

Antenna 2—

18k+10 when s is 0

18k+1 when s is 1

18k+13 when s is 2

18k+4 when s is 3

18k+16 when s is 4

18k+7 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

And, in the pilot allocation structure shown in FIG. 24, the pilotallocation indexes for the antennas may be represented in detail asfollows.

<Pilot Allocation Index for FIG. 24>

Antenna 1—

18k when s is 0

18k+9 when s is 1

18k+3 when s is 2

18k+12 when s is 3

18k+6 when s is 4

18k+15 when s is 5

Antenna 2—

18k+9 when s is 0

18k when s is 1

18k+12 when s is 2

18k+3 when s is 3

18k+15 when s is 4

18k+6 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

In addition, in the pilot allocation structure shown in FIG. 25, thepilot allocation indexes for the antennas may be represented in detailas follows.

<Pilot Allocation Index for FIG. 25>

Antenna 1—

18k when s is 0

18k+9 when s is 1

18k+6 when s is 2

18k+15 when s is 3

18k+3 when s is 4

18k+12 when s is 5

Antenna 2—

18k+9 when s is 0

18k when s is 1

18k+15 when s is 2

18k+6 when s is 3

18k+12 when s is 4

18k+3 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

In the pilot allocation structure according to the above-describedembodiment, if a preamble OFDM symbol is transmitted to a foremostportion of the subframe in a predetermined period, the pilot subcarriersmay be changed so as to be applied to a second OFDM symbol and thesubsequent OFDM symbols.

FIGS. 26 and 27 show pilot subcarrier allocation patterns in a 4-Txsystem according to another embodiment of the present invention. Thebasic pilot allocation methods of FIGS. 26 and 27 are equal to those ofthe above-described embodiments. However, in the present embodiment, thepilot subcarriers for the four antennas are allocated to be contiguousto each other in contiguous four OFDM symbol areas.

In the pilot allocation structure shown in FIG. 26, the pilot allocationindexes for the antennas may be represented in detail as follows.

<Pilot Allocation Index for FIG. 26>

Antenna 1—

18k+1 when s is 0

18k+10 when s is 1

18k+4 when s is 2

18k+13 when s is 3

18k+7 when s is 4

18k+16 when s is 5

Antenna 2—

18k+10 when s is 0

18k+1 when s is 1

18k+13 when s is 2

18k+4 when s is 3

18k+16 when s is 4

18k+7 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

Antenna 3—

18k+4 when s is 0

18k+13 when s is 1

18k+7 when s is 2

18k+16 when s is 3

18k+10 when s is 4

18k+1 when s is 5

Antenna 4—

18k+13 when s is 0

18k+4 when s is 1

18k+16 when s is 2

18k+7 when s is 3

18k+1 when s is 4

18k+10 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

In the pilot allocation structure shown in FIG. 27, the pilot allocationindexes for the antennas may be represented in detail as follows.

<Pilot Allocation Index for FIG. 27>

Antenna 1—

18k when s is 0

18k+9 when s is 1

18k+3 when s is 2

18k+12 when s is 3

18k+6 when s is 4

18k+15 when s is 5

Antenna 2—

18k+9 when s is 0

18k when s is 1

18k+12 when s is 2

18k+3 when s is 3

18k+15 when s is 4

18k+6 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

Antenna 3—

18k+3 when s is 0

18k+12 when s is 1

18k+6 when s is 2

18k+15 when s is 3

18k+9 when s is 4

18k when s is 5

Antenna 4—

18k+12 when s is 0

18k+3 when s is 1

18k+15 when s is 2

18k+6 when s is 3

18k when s is 4

18k+9 when s is 5

k: subcarrier index (k=0, 1, . . . ),

s: [OFDM symbol index] mod 6

(OFDM symbol index=0, 1, 2, . . . )

FIG. 28 shows a pilot subcarrier allocation pattern in a 4-Tx systemaccording to another embodiment of the present invention. The principledescribed in the above-described embodiments is similarly applied to thepilot pattern of FIG. 28. However, FIG. 26 shows an example that thepairs of pilots are arranged at the spacing of two subcarriers when thesubcarriers for the first antenna and the subcarriers for the secondantenna form a pair of pilots, and the subcarriers for the third antennaand the subcarriers for the fourth antenna form another pair of pilots.That is, in the present embodiment, the allocation may be performed suchthat two pairs of pilots are allocated to be contiguous to each other ornot to be contiguous to each other.

The above-described functions may be performed by a processor such as amicroprocessor, a controller, a microcontroller or an applicationspecific integrated circuit (ASIC) which is coded so as to perform thefunctions. The design, development and implementation of the code areapparent to those skilled in the art on the basis of the description ofthe present invention.

A method of allocating pilot subcarriers according to the presentinvention is applicable to an IEEE 802.16m system. As described above, abasic principle such as pilot arrangement for equally allocatingtransmission power to antennas or pilot shift pattern setting is alsoapplicable to other wireless communication systems by the same method.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of allocating pilots for use in downlinkor uplink communication in a multiple-input multiple-output (MIMO)antenna system using orthogonal frequency division multiplexing (OFDM),the method comprising: allocating first pilots for a first antenna andsecond pilots for a second antenna in time and frequency domains,wherein each one among the first pilots and each one among the secondpilots which are allocated within one subcarrier on 2 consecutive OFDMsymbols form a pair, wherein a pair allocated on specific 2 consecutiveOFDM symbols is shifted by multiple of a predetermined number ofsubcarriers in the frequency domain from a neighboring pair allocated onthe specific 2 consecutive OFDM symbols, wherein the allocated OFDMsymbol locations for the first and the second pilots of the pairallocated on the specific 2 consecutive OFDM symbols differ from thosefor the first and the second pilots of the neighboring pair allocated onthe specific 2 consecutive OFDM symbols, wherein each OFDM symbolcomprises first and second pilots, and wherein a first number of thefirst pilots and a second number of the second pilots allocated in eachOFDM symbol are the same.
 2. The method of claim 1, further comprising:allocating third pilots for a third antenna and fourth pilots for afourth antenna in the time and frequency domains, wherein each one amongthe third pilots and each one among the fourth pilots which areallocated within one subcarrier on 2 consecutive OFDM symbols form apair, hereinafter called “second type pair”, wherein a second type pairsallocated on specific 2 consecutive OFDM symbols is shifted by multipleof the predetermined number of subcarriers in the frequency domain froma neighboring pair allocated on the specific 2 consecutive OFDM symbols,and wherein the allocated OFDM symbol locations for the third and thefourth pilots of the second type allocated on the specific 2 consecutiveOFDM symbols differ from those for the third and the fourth pilots ofthe neighboring second type pair allocated on the specific 2 consecutiveOFDM symbols.
 3. The method of claim 2, wherein the allocated subcarrierlocations for the first pilots are adjacent to the allocated subcarrierlocations for the third pilots in the frequency domain, and wherein theallocated subcarrier locations for the second pilots are adjacent to theallocated subcarrier locations for the fourth pilots in the frequencydomain.
 4. The method of claim 3, wherein each OFDM symbol comprises thefirst, the second, the third and the fourth pilots, and wherein a first,a second, a third and a fourth numbers of the first, the second, thethird and the fourth pilots, respectively, allocated in each OFDM symbolare the same.
 5. The method of claim 2, wherein each of the allocatedsubcarrier locations for the third pilots shifted by two OFDM symbols isshifted by multiple of 3 subcarriers, and each of the allocatedsubcarrier locations for the fourth pilots shifted by two OFDM symbolsis shifted by multiple of 3 subcarriers.
 6. The method of claim 1,wherein a starting location of the allocated subcarrier locations forthe first pilots in a first OFDM symbol is offset by 1 subcarrier. 7.The method of claim 1, wherein any of the first to fourth pilots istransmitted by a full usage of subchannels, FUSC, permutation mode or anadaptive modulation and coding, AMC, permutation mode.
 8. A wirelesscommunication system using orthogonal frequency division multiplexing(OFDM) for downlink or uplink communication, the wireless communicationsystem comprising: a multiple-input multiple-output (MIMO) antenna; anOFDM modulator operably connected to the MIMO antenna; and a processoroperably connected to the OFDM modulator, wherein the processor isconfigured to allocate first pilots for a first antenna and secondpilots for a second antenna in time and frequency domains, wherein eachone among the first pilots and each one among the second pilotsallocated within one subcarrier on 2 consecutive OFDM symbols form apair, wherein a pair allocated on specific 2 consecutive OFDM symbols isshifted by multiple of a predetermined number of subcarriers in thefrequency domain from a neighboring pair allocated on the specific 2consecutive OFDM symbols, wherein the allocated OFDM symbol locationsfor the first and the second pilots of the pair allocated on thespecific 2 consecutive OFDM symbols differ from those for the first andthe second pilots of the neighboring pair allocated on the specific 2consecutive OFDM symbols, wherein each OFDM symbol comprises first andsecond pilots, and wherein a first number of the first pilots and asecond number of the second pilots allocated in each OFDM symbol are thesame.
 9. The wireless communication system of claim 8, wherein theprocessor is further configured to allocate third pilots for a thirdantenna and fourth pilots for a fourth antenna in the time and frequencydomains, wherein each one among the third pilots and each one among thefourth pilots allocated within one subcarrier on 2 consecutive OFDMsymbols form a pair, hereinafter called “second type pair”, wherein asecond type pair allocated on specific 2 consecutive OFDM symbols isshifted by multiple of a predetermined number of subcarriers in thefrequency domain from a neighboring pair allocated on the specific 2consecutive OFDM symbols, and wherein the allocated OFDM symbollocations for the third and the fourth pilots of the second type pairallocated on the specific 2 consecutive OFDM symbols differ from thosefor the third and the fourth pilots of the neighboring second type pairallocated on the specific 2 consecutive OFDM symbols.
 10. The wirelesscommunication system of claim 9, wherein the allocated subcarrierlocations for the first pilots are adjacent to the allocated subcarrierlocations for the third pilots in the frequency domain, and wherein theallocated subcarrier locations for the second pilots are adjacent to theallocated subcarrier locations for the fourth pilot subcarriers in thefrequency domain.
 11. The wireless communication system of claim 10,wherein each OFDM symbol comprises the first, the second, the third andthe fourth pilots, and wherein a first, a second, a third and a fourthnumbers of the first, the second, the third and the fourth pilots,respectively, allocated in each OFDM symbol are the same.
 12. Thewireless communication system of claim 9, wherein each of the allocatedsubcarrier locations for the third pilots shifted by two OFDM symbols isshifted by multiple of 3 subcarriers, and each of the allocatedsubcarrier locations for the fourth pilots shifted by two OFDM symbolsis shifted by multiple of 3 subcarriers.